With my old laptop about to die (screen playing up, power management keeps slowing the cpu down etc), I've had to give in and get a new laptop.

The one I've ended up with is a Dell Inspiron 9400, which comes with the ATI Radeon Mobility X1400. Being very new, your only choice is to use the binary fglrx drivers.

The laptop itself has a nice widescreen LCD, running at 1440x900. My second screen runs at 1280x1024. My initial attempts with the ATI drivers failed to work properly - either I couldn't move windows between the screens (with dual head mode), or the second screen came up in a random mode (1156x864), which the screen really didn't like and looked crap+distorted (with big desktop).

Then someone pointed me at a new feature with the 8.28.8 ATI driver - pairmode. Documentation on this was pretty much non existent, but with a bit of poking I got it working.

It seems that you can only define the one pairmode, and for now X will come up in the random resolution it want. However, you can use xrandr to switch it into the mode you really wanted. Also, if the second screen is larger in any dimension than the first, then you'll need to define a virtual entry to get the sizes right.